For example, as a means for detecting deformation of a member and a magnitude of a load acting on the member, a flexible sensor using an elastomer or a resin has been proposed. The sensor has a sensor body in which conductive filler particles are mixed in the elastomer or the resin. For example, in a sensor body disclosed in Patent Documents 1 and 2, spherical conductive filler particles are mixed at a high filling rate in an elastomer or a resin. Therefore, in the sensor body in a state in which a load is not applied (which may be referred to as a “no-load state” as appropriate in the following), 3-dimentional conductive paths are formed by contact among conductive filler particles. Therefore, the sensor body has a high electrical conductivity in the no-load state.
FIG. 17 includes schematic diagrams enlarging a portion of conductive filler particles in a sensor body. FIG. 17(a) illustrates a no-load state before a bending deformation; FIG. 17(b) illustrates a state immediately after the bending deformation; and FIG. 17(c) illustrates a state further after the state immediately after the bending deformation. As FIG. 17(a) illustrates, a sensor body 900 has a matrix resin 901 and conductive filler particles 902. In the sensor body 900, a conductive path P1 is formed by contact among the conductive filler particles 902. When a load is applied to the sensor body 900, the sensor body starts a bending deformation. As FIG. 17(b) illustrates, along with the start of the bending deformation, the matrix resin 901 is stretched in a left-right direction in the figure. This causes the conductive filler particles 902 to repel each other and the contact state of the conductive filler particles 902 to change. As FIG. 17(c) illustrates, when the sensor body 900 bends further, the matrix resin 901 is stretched further. This causes the contact among the conductive filler particles 902 to be broken and the conductive path P1 to be cut off. As a result, electrical resistance increases. When the applied load is removed, the sensor body 900 is restored to its original state (the state illustrated in FIG. 17(a)) by an elastic restoring force of the matrix resin 901. Thus, according to the sensor of Patent Documents 1 and 2, a deformation can be detected based on an increase in the electrical resistance of the sensor body.